The invention relates to a DC/DC converter having a plurality of two-pole inverters connected in parallel or in series, just as many transformers and just as many two-pole rectifiers connected in parallel or in series. One inverter is connected to a primary side of one transformer and one rectifier is connected to a secondary side of one transformer. Furthermore, the inverters are connected to a control which is provided for frequency-synchronous actuation of the inverters, with a 180°/n phase shaft, with the aid of a control signal, n indicating the number of transformers. In addition, the invention relates to an AC/DC converter.
A DC/DC converter (also “direct current chopper”) converts a constant input voltage by periodic switching so that a different arithmetic mean voltage value is produced at the output. The basic types are step-down converter (output voltage lower than the input voltage), step-up converter (output voltage higher than the input voltage) and inverter (output voltage has a negative sign compared with the input voltage).
An AC/DC converter converts an input AC voltage into an output DC voltage, the DC voltage as a rule differing from the effective value of the AC voltage. AC/DC converters in the form of power units which, for example, permit the supply of electronic devices on an AC voltage network are widely used. In addition, AC/DC converters in the form of rectifiers are encountered.
DE 100 51 156 A1, which describes a converter circuit for medium-frequency transformational energy transmission by means of a transformer connected on the primary side and secondary side to power electronic rectifiers, is mentioned as a further example of a converter. At least two rectifiers controllable independently of one another and connected to in each case a partial winding of a single-core transformer are synchronized alternately, whereby they produce either bipolar voltage pulses in succession or monopolar voltage pulses of opposite polarity alternately, in order to increase, for example, the power density of the transformer.
EP 1 227 571 A2 furthermore discloses a DC/DC converter having converter parts, transformers and rectifier parts. Two sets of converter parts comprise two pairs of first switching elements and two further pairs of second switching elements, which are connected in a full bridge configuration. Series capacitors are arranged between the converter parts and the transformers. The switching points of the first switching elements are phase-shifted by a ⅓ n period relative to the switching points of the second switching elements. The switching points of the switching elements of the converter parts on the other hand are phase-shifted by ½ n periods relative to one another.
Finally, EP 1 391 982 A2 discloses a circuit comprising a pair of converter circuit parts for converting a supply voltage of a DC voltage source into an AC voltage. This is effected by two pairs of switching elements in full bridge configuration, which are arranged parallel to the DC voltage source and rectifier part and are led via transformers to the outputs of each converter circuit part. Series capacitors are arranged as a smoothing unit between the converter circuit parts and the transformers. The secondary sides of the transformers of a rectifier part of each of these smoothing units are connected in series in N groups of smoothing units. Furthermore, the secondary sides of the other rectifier parts are also connected in series in the N groups of smoothing units.
DC/DC converters are to be found in almost all areas of industry, for example in the automotive industry, if the on-board voltage of typically 12 volt is to be converted into a different DC voltage, in battery-operated devices, if the battery or accumulator voltage is not the same as the voltage required by the electronic circuit, or in electrical drive technology.
Not least, DC/DC converters are also used in combination with an AC/DC stage, an AC voltage (in Europe typically 230 V AC) being converted into an intermediate circuit voltage (DC voltage) and this intermediate circuit voltage, which often cannot be generated as high or as low as desired, then being brought to a desired level by a DC/DC converter.
An important field of use of AC/DC converters is, inter alia, also the charging of batteries or accumulators at the power supply, which is becoming increasingly important owing to the continuously increasing mobility of people and the associated operation of mobile electrical and electronic devices. Not least, electrically driven motor vehicles also set completely new requirements with regard to battery chargers, since on the one hand the charging should be possible both at the single-phase power supply (customary in the household), owing to the greater power density and hence faster charging, but on the other hand also at the three-phase power supply. Associated therewith are differences in the mains voltage (in Europe 230 V AC in the single-phase power supply and 400 V AC in the three-phase power supply) which—because in fact the battery voltage is constant—must be taken into account in the circuit technology. Furthermore, a comparatively high energy content (which in the end determines the range of the vehicle) should be transferred in as short a time as possible from the power supply to the battery.
The power of the DC/DC converters and/or AC/DC converters, which in some cases is considerable, unfortunately often leads in the case of a conventional design to comparatively high power ripples on the load (e.g. battery). However, the feedbacks to the supply side, i.e. for example into the supplying power network, can also be problematic. Such feedbacks may occur, for example, in the case of transient processes which are due, for example, to switching processes or nonlinear consumers, in particular to components of the power electronics, such as, for example, transistors and thyristors.
It is an object of the invention to provide a DC/DC converter and/or AC/DC converter in which power ripples on a load connected to the converter and/or supply-side feedbacks are reduced.
Accordingly, a DC/DC converter according to the invention and of the type mentioned at the outset additionally comprises the feature that leakage inductances of the transformers, with capacitors of the inverters and/or capacitors of the rectifiers, form in each case a resonating circuit whose resonant frequency is substantially twice as large as a clock frequency of the control signal.
Accordingly, an AC/DC converter according to the invention comprises a DC/DC converter according to the invention which has an AC/DC stage on the supply side.
By means of this circuit—as will be shown later—power ripples on the load and/or supply-side feedbacks (e.g. mains feedbacks) are effectively reduced. The converters according to the invention are therefore suitable in particular for the purposes mentioned at the outset, but their use is by no means limited to these areas. Owing to the reduced mains feedbacks, in particular very powerful battery chargers (e.g. for electric vehicles) can thus be realized, in which power ripples on the load are at least reduced—if not avoided—and the specifications of the power supply companies with regard to the maximum mains feedbacks are fulfilled.
According to the invention, sinusoidal AC voltages which are superposed on a DC voltage and are phase-shifted by 180° result at the capacitors. This has the advantage that the switching of the rectifier or inverter takes place at the crossover of the current, which practically eliminates the switching losses and reduces the electromagnetic disturbances to a minimum. In the case of series connection of the inverters or rectifiers, the phase-shifted AC voltages compensate one another, resulting in a practically ripple-free DC voltage. The term “substantially” is defined in this context by that level of an AC voltage superposed on the DC voltage which is tolerable in terms of engineering. If the leakage inductance of the transformers is too small, it can of course be increased to the desired value by an additional coil in series with the primary and secondary winding.
For resonance, the parallel connection of the leakage inductance and main inductance of the transformers is decisive. Since the main inductance is however about 100 times greater than the leakage inductance in most applications, the main inductance acting in parallel with the leakage inductance is generally negligible in practice. If, however, the actual ratios differ decisively from those described above, it may, however, also be appropriate to take into account the main inductance.
Furthermore, it should be pointed out that a “converter” in the context of the disclosure may be understood as meaning a rectifier, an inverter or a bidirectional converter. A “rectifying element” may be understood, for example, as meaning a diode, a transistor (e.g. an MOSFET or an IGBT), a thyristor or another rectifying element. Furthermore a “switching element” may be understood, for example, as meaning a transistor (e.g. once again an MOSFET or an IGBT), a thyristor or another switching element. Accordingly, there are passive rectifiers, active rectifiers or active inverters. Finally, “batteries” are to be understood as meaning chargeable batteries, accumulators or other power-storing media.
Advantageous configurations, versions, and further developments of the invention shall be understood from the following description and drawings.
It is advantageous if the DC/DC converter has two inverters, two transformers and two rectifiers, since this gives a comparatively simple design.
It is also advantageous if the transformers have separate cores since in this way mutual influencing thereof can be more or less ruled out.
It is advantageous if one capacitor each is connected in parallel to one inverter each and/or one capacitor each is connected in parallel to one rectifier each. In this way, power ripples and/or feedbacks to the supply side can be particularly readily reduced since the already comparatively smooth DC voltage is further smoothed.
It is furthermore advantageous if a common capacitor is connected in parallel to the inverters and/or a common capacitor is connected in parallel to the rectifiers. This variant of the invention, too, ensures a particularly good reduction of power ripples and/or feedbacks to the supply side, but the number of capacitors is reduced in this variant. The circuit is therefore technically simpler as a whole.
It is advantageous if one rectifying element is connected antiparallel to one switching element each of an inverter and one switching element is connected antiparallel to one rectifying element each of a rectifier. The DC/DC converter can then convert electrical energy not only in one direction but also in the opposite direction and in principle therefore permits bidirectional operation. Suitable switching elements are, for example, components from power electronics, in particular transistors or thyristors. The abovementioned converters, i.e. passive rectifiers, active rectifiers or active inverters, can therefore also be used in any desired combination, i.e. for example a passive rectifier combined with an active inverter.
It is also advantageous if the control signal is a square-wave signal. This signal can easily be generated and is therefore particularly suitable for the DC/DC converter according to the invention. A symmetrical square-wave signal, i.e. a square-wave signal with 50% pulse duty factor, is particularly advantageous. The use of other signal shapes, for example delta or sine shapes, is, however, not ruled out thereby.
It is furthermore advantageous if the inverters and/or the rectifiers are connected in parallel or can be connected in parallel and coils are arranged between the inverters and/or the rectifiers. In this way, in spite of the parallel connection, the resonating circuits described above are enabled to oscillate in phase opposition with their resonant frequency, so that, with retention of the mode of operation described above, the direct currents sum but the ripple currents subtract at the common connection of the two coils. In this way, a compensation of the voltage ripple can be achieved even when the inverters and/or the rectifiers are connected in each case in parallel on both sides of the transformers.
In the case of the parallel connection mentioned, it is advantageous if the coils are coupled. In this way, the size of the coils can be substantially reduced. The two coupled coils may furthermore be considered as a transformer or may be a part thereof. If a transformer is provided, then advantageously only one component is required for compensating the voltage ripple, instead of two components. Furthermore, the use of the transformer gives rise to energy advantages compared with separate coils. In a further advantageous configuration of the invention, an autotransformer is provided for this purpose.
Finally, it is advantageous if the AC/DC converter is provided for bidirectional operation by a combination of a rectifier with an antiparallel inverter and vice versa (in this case also in the AC/DC stage). This can optimally be used, for example, in the solar power sector in that, in the case of sunlight and an excess of direct current, electric power firstly can be stored in batteries but secondly can also be delivered to the alternating current mains. If darkness then coincides with a demand for alternating current, firstly alternating current can be taken from the mains (if the mains is capable of delivering) and secondly current from the batteries can be converted into alternating current and hence serve the installation as an AC voltage source. Furthermore, an electric vehicle (not used for travelling) with its battery can accordingly be operated as a power buffer for an alternating current mains.
It should be made clear here that the invention relates not only to converters for battery chargers but to converters generally. Moreover, the battery chargers relate not simply to applications in motor vehicle construction or in solar technology but generally to battery chargers. The person skilled in the art will easily establish here the suitability of the invention also for other fields of use.
The above configurations and further developments of the invention can be combined in any desired manner.